1. Please state project objectives and what work was done this quarter to address them: The overall goal of the project is to develop fertilization strategies to best match nutrient supply and demand, and develop recommendations for optimal nutrient application timing as compared to a simple constant supply, which will improve fruit yield, quality, and reduce fruit drop. A)Objective 1) Test if a reduced N-P-K nutrient supply in the fall is safe for sustaining HLB-affected citrus, and whether it can improve fruit quality to facilitate earlier maturity / harvesting and reduce fruit drop: We established two replicated field trials with 10 fertilizer rate and timing combinations to evaluate the growth, fruit development, maturity, yields and quality of 5-year old Valencia and Hamlin trees on X-639 rootstock. The first fertilizer of about 20% annual N was applied in the first week of March. Soil samples were collected from each plot for nutrient analysis, vacuum lysimeters were installed under trees to sample nutrient leachates, and minirhizotrons were installed in the field to measure root growth. Soil P tests in the site were hight to very high, so we omitted P fertilizer from the blended fertilizer in order to deplete soil P reserves down to nominal levels. The third field trial was established on 5-year old Sugarbelle trees, and sub-objectives were modified to focus on improving fresh-fruit characteristics, particularly peel thickness, fruit size, color break, juice content, and internal quality (brix, acid). The basis for this refocusing was in response to the Sugarbelle “soft fruit” and quality reports of the 2021/22 season. Objective 2) Develop an optimized, practical fertilizer timing management profile to boost fruit quality and reduce fruit drop for HLB-affected citrus based in part on the sigmoidal nutrient demand curve defined by four physiological growth phases (0=bloom/fruit set; 1=cell division; 2=cell enlargement; 3=maturation): We are using the existing Citrus Diagnosis smartphone AI app as a baseline, and exploring improved methods for classifying foliar nutrient deficiency symptoms such as magnesium, iron, manganese and zinc so that the intensity of deficiency can be quantified from the severity and extent of chlorotic tissue on an area basis. Semantic segmentation models were trained on leaf images to quantify chlorosis and generative adversarial networks were tested to generate hypothetical synthetic nutrient symptoms to include in a comprehensive fertilizer management and nutrient modeling app. B) New developments: sub-objectives concerning the Sugarbelle trial were refocused on solving the current fruit quality issues, but temporal fertilization strategies are still at the core of the trial. C) Issues: We are concerned about the low fruit set in the city block. The trees flowered in January, and the bloom was apparently damaged by the January 30 freeze. The trees produced some recovery bloom about a month later, but it was small. 2. Please state what work is anticipated for next quarter: The second of four fertilizer applications will be made in the first week of April, and the third will be in mid May. Some treatments in the 10-treatment structure will not receive fertilizer at every date, or receive variable amounts of fertilizer in order to construct 10 different fertilization strategies (supply curves) over time. Soil, lysimeter and leaf sampling, processing and analysis will be ongoing, as will tree size and fruit measurement. The first Aerobotics drone survey will fly in early May, to evaluate tree health and size indicators. 3. Please state budget status (underspend or overspend, and why): Grove chargebacks for 3 acres of field experiments were initiated, partial salaries of 3 personnel were allocated for the new efforts of this project, and supplies for installing minirhizotrons were purchased. There may be a temporary underspend due to system lag in reporting the modified salary allocations to this project, which was done recently.